TAS3103DBT datasheet(97/146 Pages) TI

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3−37

INPUT +

6.0206

[

*7.5 ) 24.0824] + 2.754277

+ 010.1100_0001_0001_1000_0100_110

+ 0x000001608C26 in 25.23 format

For input levels above the T2 threshold, the transfer function exhibits a 1:1.1 expansion. For input levels below T2,

the transfer function exhibits a 2:1 compression. Also, by definition, it is seen that there is no discontinuity in the

transfer function at T2. When the 2:1 compression curve in region 1 intersects the T1 threshold level, the output level

is 7.5 dB above the 1:1 transfer, an offset value identical to O1. Thus, there is no discontinuity at T1. For input levels

below T1, the transfer function exhibits a 1:1.05 expansion.

DRC rms input levels below −192 dB fall below the 32-bit precision of the DRC input (32 bits x −6 dB/bit = −192 dB).

This means that for levels below −192 dB, the DRC sees a constant input level of 0, and thus the computed DRC gain

coefficient remains fixed at the value computed when the input was at −192 dB. The transfer function then has a 1:1

slope below the –192-dB input level and is offset from the 1:1 transfer curve by the offset present at the –192-dB input

level.

The change from a 1:1.05 expansion to a 1:1 transfer below −192 dB is the result of 32-bit dynamic range saturation

at the DRC input. This type of saturation always occurs at a DRC input level of −192 dB. However, the input level at

which this type of saturation occurs depends on the channel gain. For this example, the saturation occurs at an input

level of −168 dB (−192-dB DRC input + 48 dB 8-bit headroom –24-dB mixer gain into DRC).

3.7.3.2 Example 2—Compression/Expansion/Compression Transfer Function With Maximum Gain

Saturation and 32-Bit Dynamic Range Saturation

The transfer function parameters for this example are given in Table 3−5. In setting the threshold levels it is assumed

that the net processing gain between the SAP input and the DRC is 0 dB. This is the same as Example 1 except that

the gain of the mixer into the DRC is set to 1 instead of 24. Because of the 8-bit headroom in the 48-bit DAP word,

the upper eight bits of the 32-bit DRC input word are zero, resulting in 0-dB signal levels at the SAP input being seen

as –48-dB signal levels at the DRC.

Figure 3−23 shows the DRC transfer function resulting from the parameters given in Table 3−5. At threshold level

T2 (−70 dB), O2 specifies a boost of 30 dB. But the signed 5.23 formatted gain coefficient only provides a 24-dB boost

capability (5 integer bits = Sxxxx

≥ 24 × 6 dB/octave = 24 dB). Internally, the DRC operates in 48-bit space and thus

computes a 30-dB boost. But the 5.23 formatted gain coefficient saturates or clips at 24 dB. The transfer curve thus

resides 24 dB above the 1:1 transfer curve at T2.

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